1. Field of the Invention
The present invention relates to a dispersion liquid of metal oxide fine particles for use in producing a molding product that requires high transparency.
2. Description of the Related Art
Recently, optical materials are extensively studied, but especially in the field of lenses, a demand continues to exist for optical materials excellent in high refractivity, heat resistance, transparency, easy-to-mold property, lightness, chemical resistance, solvent resistance, and the like.
Plastic lenses are lightweight and unbreakable compared to lenses made of an inorganic material such as glass, and plastic can be formed into lenses having various shapes, and accordingly the plastic lenses are widely and rapidly prevailing not only in eyeglasses but also in optical materials such as lenses for a portable camera and a pickup lens in recent years.
In addition, it is required that the material itself have a high refractive index, aiming at thinning of a lens and downsizing of an image pickup device. For example, techniques have extensively been studied such as a technique in which a sulfur atom is introduced into a polymer (Japanese Patent Application Laid-Open (JP-A) Nos. 2002-131502 and 10-298287), a technique in which a halogen element or an aromatic ring is introduced into a polymer (JP-A No. 2004-244444). However, a plastic material still remains unknown having sufficient refractive index, excellent transparency and light resistance, and capable of displacing glass. It is also to be noted that in an optical fiber or a light guide, materials having different refractive indexes are used in combination and materials having a distribution of refractive index are used. To cope with these materials whose refractive index tends to vary depending on the sites, a technique for controlling arbitrarily a refractive index has been demanded.
Since it is difficult to heighten a refractive index with organic materials alone, a method has been reported in which a resin is made to be highly refractive by dispersing an inorganic material having a high refractive index in the resin matrix (JP-A No. 2003-73559). Meanwhile, in order to lower the decrease of transmitted light by Rayleigh scattering, it is preferable to homogeneously disperse inorganic fine particles having a particle size of 15 nm or less into a resin matrix. However, since a primary particle having a particle size of 15 nm or less is extremely easy to aggregate, it is very difficult to disperse it homogeneously in a resin matrix. Moreover, in view of the decrease of transmitted light in a light path length that corresponds to the thickness of a lens, the addition amount of an inorganic fine particle has to be limited. Thus, it has so far been unable to disperse fine particles into a resin matrix at a high concentration without sacrificing the transparency of a resin.
There are other reports relating to a molding product containing mainly a thermoplastic resin in which ultra fine particles having number average particle diameter of 0.5 nm to 50 nm are dispersed, the molding product being a complex composition molding product in which an average double refraction index at 1 mm of light path length is 10 nm or less (JP-A No. 2003-147090); and an organic/inorganic complex material composition and optical parts made of it, the composition containing a thermoplastic resin having a refractive index represented by a specific numerical formula and Abbe number and inorganic fine particles having a specific average particle diameter and a specific refractive index (JP-A Nos. 2003-73563 and 2003-73564). These techniques also disperse inorganic fine particles in a resin, but cannot achieve sufficient performance in a standpoint of dispersing fine particles at a high concentration in a resin matrix without reducing transparency of the resin.
Although materials to be used in preparing an inorganic particle greatly vary depending on a method of synthesis or a solvent to be used, in synthesizing, for example, an oxidized product by hydrolyzing a metal oxide in an aqueous solution, metal halides or metal oxidized halides is often used in terms of cost and solubility. Of the halides, chlorides are often used in terms of solubility. In the case where a metal oxide is synthesized by hydrolyzing a metal alkoxide, hydrochloric acid is often used as an acid. If the halogens are excessively present in a solution, the inorganic fine particles easily aggregate, making it impossible to produce highly transparent dispersion liquid.
Japanese Patent (JP-B) No. 3524342 refers to the control of chlorine content in aqueous titanium dioxide dispersion liquid and describes that it is better to contain 1000 ppm to 10,000 ppm level of chlorine to enhance adhesive property in forming and sintering a coated thin film. In this document, stabilization is made by removing chlorine ions with electrodialysis and incorporating polyvinylalcohol. The average particle diameter of titanium dioxide is 0.01 μm to 0.1 μm. Although the effect of Rayleigh scattering is marked in this region of particle diameter, it is not problematic even in slightly large particles or even when the particles are aggregated by chlorines because they are used as a thin film. However, the method is not applicable to a molding product having a thickness level of several hundred micrometers to several millimeters, in which light scattering effect is marked, and transparency is too low.
An average primary particle diameter of the inorganic fine particles greatly affects the Rayleigh scattering, provided that the primary particles are dispersed individually without aggregating. However, fine particles are likely to aggregate when the average primary particle diameter is small. Even when the average primary particle diameter is very small, a secondary particle diameter of the aggregates thereof can be large, lowering the transparency by Rayleigh scattering. Here, the term “average primary particle diameter” refers to the average diameter of a circle corresponding to the photograph of a particle obtained by the observation with the use of a transmittance electron microscope (TEM). Thus, it is clear that the transparency of the dispersion liquid or a composite molding product cannot be determined only from the average primary particle diameter.
As a method of determining a particle diameter in a state of a dispersion liquid, a dynamic scattering method is commonly used. If each metal oxide fine particle perfectly stands alone as a primary particle, the particle diameter corresponds to a primary particle diameter, but if the particles are aggregated, the particle diameter corresponds to a secondary particle diameter. Whether a particle diameter obtained by a dynamic scattering method is primary or secondary particle diameter can be determined by the comparison with the data from TEM observation.
The state of the aggregates of metal oxide fine particles depends on the conditions adopted at particle preparation. In the case where the synthesis is conducted at a high temperature aiming at enhancing crystallization or increasing the yield, particles tend to aggregate in such a condition. Likewise, aggregation tends to occur in the condition that the concentration of metal oxide fine particles is extremely high. It is also known that a high concentration of halogen elements in a dispersion liquid results in the aggregation. The required primary particle size or the secondary particle size varies depending on performance of a device that is produced by using the particles, and is regulated by a corresponding synthesis method.
For example, in the case of a transparent conductive film which is produced by coating metal oxide fine particle dispersion liquid on a film, drying it, and firing the dried coating, the thickness of the film is ordinary in the level of several hundred nanometers to several micrometers. As for such a relatively thin film, enough transparency can be secured even when a primary particle diameter or secondary particle diameter exceeds, for example, 30 nm. However, as to an optical device such as lenses for a digital camera, which requires high transparency and thickness of several millimeters, the effect of Rayleigh scattering clearly appears, and accordingly it is necessary for securing transparency to use an inorganic fine particle which are sufficiently small but not in an aggregated state.
The halogen elements contained in a dispersion liquid of metal oxide fine particles are originated from an acid which serves as a catalyst for the synthesis or a compound which is a raw material for the metal oxide fine particles. Therefore, a concentration of halogen elements in the dispersion liquid varies depending on the concentration of the metal oxide fine particles or the acid catalyst to be used. The amount of the halogen elements can be controlled by, for example, ultrafiltration or electrodialysis. As mentioned above, when the concentration of halogen elements is high, the metal oxide fine particles easily aggregate by a salting-out effect and form large secondary particles, meaning that they cannot be used for producing optical devices which require high transparency. Meanwhile, if ultrafiltration is used to reduce the halogen content, the transparency of a sol can be improved to some level of the concentration in accordance with the decrease of halogen content, but the metal oxide fine particles begin to aggregate when pH of the solution greatly exceeds the pH range where each individual particle can be dispersed stably.
If, for example, nitric acid or sulfuric acid is added to regulate the pH, transparency of the sol decreases. But when a carboxylic acid is used in place of nitric acid or sulfuric acid, pH is kept within the pH range corresponding to the range where the fine particles can be present stably, to thereby reducing halogen content without deteriorating the transparency. This is because the carboxyl group of the carboxylic acid adsorbs to the surface of metal oxide fine particles and plays a roll as a dispersant, preventing the particles from aggregation so that the sol is stable with retaining high transparency.
When a carboxylic acid is used as an acid catalyst to prepare metal oxide fine particles, the carboxylic acid may be used from the outset. However, since a carboxylic acid is less effective as a catalyst, hydrochloric acid or nitric acid is often used in many cases. In the latter case, after the metal oxide fine particles are prepared, it is necessary to substitute them with a carboxylic acid.